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Over a century ago it was discovered that invisible light rays in the ultraviolet region of the light spectrum caused the skin to darken. As scientists studied ultraviolet light, they divided this region into three groups, UVA (320-400 nm), UVB (280-320 nm), and UVC (200-280 nm). Nanometers, or nm for short, define the wavelength of the ultraviolet light; one nanometer is about 1/100,000 the thickness of a human hair. The primary effect of UVA light on the skin is called pigmentation, or darkening of the skin.
The primary effect of UVB on the skin is called erythema, or reddening of the skin. Erythema begins shortly after exposure to UVB, and varies depending upon the intensity and length of the exposure. How much redness is also determined by how much UVB is coming from which wavelengths; for example, UVB at 290 nm produces 100 times more erythema than UVB at 320 nm! At the same time as erythema is taking place, the UVB causes melanin to form just beneath the surface of the skin. The melanin is oxidized and turned brown by the UVA rays given off by the tanning lamps.
Thus, the tanning process involves the relatively quick reddening of the skin, followed by a slower forming, but longer lasting browning of the skin. Without some exposure to UVB rays, the tanning process cannot take place. The key is providing sufficient amounts of UVB to properly stimulate melanin, followed with effective amounts of UVA to complete the tanning process.
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